Field of the Disclosure
The present disclosure relates to a red phosphorescent compound and an organic light emitting diode (OLED) device and more particularly to a soluble red phosphorescent compound having excellent color purity and high brightness and emitting efficiency and an OLED device using the same.
Discussion of the Related Art
Recently, the requirements for flat panel display devices, such as a liquid crystal display device and a plasma display panel, have increased. However, these flat panel display devices have relatively slow response time and narrow viewing angle in comparison to the cathode ray tube (CRT).
An organic light emitting diode (OLED) device is one of next-generation flat panel display devices being capable of resolving the above problems while occupying a small area.
Elements of the OLED device can be formed on a flexible substrate such as a plastic substrate. In addition, the OLED device has advantages in the viewing angle, the driving voltage, the power consumption and the color purity. Moreover, the OLED device is adequate to produce full-color images.
Generally, the emitting diode of the OLED device includes the anode, the hole injecting layer (HIL), the hole transporting layer (HTL), the emitting material layer (EML), the electron transporting layer (ETL), the electron injecting layer (EIL) and the cathode.
The OLED device emits light by injecting electrons from a cathode as an electron injection electrode and holes from an anode as a hole injection electrode into an emission compound layer, combining the electrons with the holes, generating an exciton, and transiting the exciton from an excited state to a ground state.
The emitting principle may be classified into fluorescent emission and phosphorescent emission. In the fluorescent emission, the organic molecule in the singlet exited state is transited to the ground state such that light is emitted. On the other hand, in the phosphorescent emission, the organic molecule in the triplet exited state is transited to the ground state such that light is emitted.
When the emitting material layer emits light corresponding to an energy band gap, the singlet exciton having 0 spin and the triplet exciton having 1 spin are generated with a ratio of 1:3. The ground state of the organic material is the singlet state such that the singlet exciton can be transited to the ground state with emitting light. However, since the triplet exciton can not be transited with emitting light, the internal quantum efficiency of the OLED device using the fluorescent material is limited within 25%.
On the other hand, if the spin-orbital coupling momentum is high, the singlet state and the triplet state are mixed such that an inter-system crossing is generated between the singlet state and the triplet state and the triplet exciton also can be transited to the ground state with emitting light. The phosphorescent material can use the triplet exciton as well as the singlet exciton such that the OLED device using the phosphorescent material may have 100% internal quantum efficiency.
Recently, iridium complex, e.g., bis(2-phenylquinoline)(acetylacetonate)iridium(III)(Ir(2-phq)2(acac)), bis(2-benzo[b]thiophene-2-yl-pyridine)(acetylacetonate)iridium(III)(Ir(btp)2(acac)) and tris(2-phenylquinoline)iridium(III)(Ir(2-phq)3), as a dopant has been introduced.
To obtain high current emitting efficiency (Cd/A) with the phosphorescent material, excellent internal quantum efficiency, high color purity and long life-time are required. Particularly, referring to FIG. 1, as the color purity becomes higher, i.e., higher CIE(X), the color sensitivity becomes bad. As a result, with the high internal quantum efficiency, it is very difficult to obtain emitting efficiency. Accordingly, new red phosphorescent compound having excellent color purity (CIE(X)≧0.65) and high emitting efficiency is required.
On the other hand, in addition to the above iridium complex, another complex, for example 4,4-N,N-dicarbazole-biphenyl (CBP), is used as the red phosphorescent compound. However, these compounds do not have desirable solubility in solvent such that it is impossible to form an emitting layer by a solution process. Since the emitting layer should be formed by a deposition process, a fabricating process is very complex and a process efficiency is very low. In addition, wasted material is very much in the deposition process such that production costs are increased.